The present invention relates to free-space optical communication systems and may be used for wireless/fiberless two-way information transfer between remote objects, including situations with many objects participating in point-to-point or point-to-multipoint information exchanges.
Through-out the developed and developing world, modem society continues to create exponentially increasing demands for digital information and the communication of such information between data devices. A variety of optical fiber and wired technologies now provide high bandwidth with attendant high data rates for communications to customer premises, but installation of such facilities is not practical in all locations. In many major population centers, installation of a new physical facility of this type requires underground installation with a high construction cost. The construction and the attendant requirement for local government approval impose considerable time delays. In many instances, the actual available capacity through long delayed deployments lags far behind the ever-increasing demand. Radio frequency (RF) wireless solutions reduce the time, complexity and cost of installation, but those solutions are inherently limited by their use of shared RF spectrum. As the number of users on a given piece of spectrum grows, the average capacity available to any one user declines.
Free-space optical communications systems offer two-way information transfer between remote objects without the use of wires and/or optical fibers. Because such systems can implement point-to-point links to the individual customer premises, such systems are not subject to the limits of shared capacity, as in the existing RF wireless technologies. Free-space optical communications systems may implement network (mesh) technologies for information transfer or point-to-multipoint technology for a two-way information exchange.
One known optical communication system uses two terminals, each which includes a transmitter in a form of a light source with modulation means, and a receiver (see U.S. Pat. No. 4,960,315). This known system is used as a backup, allowing restoration of communication upon disruption of a section of fiber optic connectivity. The disclosed system provides communication via an optical radiation beam propagating in free space. A drawback of the known system, however, is that its application is limited because it provides communication only via a single channel (one transmitter to one receiver) and hence has a low transmission capacity.
A known multi-channel optical communication system uses several transmitting devices implemented as lasers with modulation means, a transmitting medium implemented as an optical fiber transparent for the laser radiation, and several receiving devices (see U.S. Pat. No. 5,589,968). This system achieves an increase in transmission capacity due to spectral, time, and code multiplexing of channels. A drawback of this known system, however, is its limited application and high cost. This is because optical fiber is used as the optical radiation transmission medium. As noted, use of optical fiber requires expensive installation by laying optical cable, e.g. underground or on masts and typically requires government permissions and/or rights-of-way, which adds considerable further expenses.
A multi-channel free-space optical communication network is known based on use of a multi-address distributing device (router) comprising a scanner which can alternately point the light beam through the atmosphere at one or several receiving devices from a multitude of receiving devices, and a lens array where each lens provides collimation of the beam pointed at the corresponding to it receiving device or at several receiving devices (see U.S. Pat. No. 5,786,923). The known communication network may also use radiation of different wavelengths, wherein each wavelength is pointed at the corresponding receiving device by means of a dispersing optical device. Also, this known network may use spectral or time division of channels formed within the light beam dedicated to each of the receiving devices, in the same manner as when transmitting information via an optical fiber.
A drawback of the known free-space multi-channel system is that it does not provide for maximization of information transmission rate through the atmosphere. This drawback is critical, because the free-space channel providing information transmission through the atmosphere is, as a rule, the xe2x80x9cbottleneckxe2x80x9d in any communication network consisting of fiber optic and free-space (atmosphere) links.
The lower capacity of a free-space path in the atmosphere relative to a fiber optic path results from the presence of excessive noise arising from background light (primarily sunlight). Other factors that reduce capacity include the considerable losses of signal power. Substantial signal losses result from absorption and scattering in the atmosphere. Also, there are xe2x80x9cgeometrical lossesxe2x80x9d caused by widening of the light beam, during passage through the atmosphere, up to diameters exceeding the light receiving aperture size, i.e. so that substantial portions of the radiant energy are not directed on the intended receiver. To mitigate these drawbacks, the use of the free-space atmospheric channels requires additional methods of channel multiplexing (in comparison to fiber optic systems).
A free-space optical communication system, known from the Description to the Japanese patent application No. 06303198, Oct. 10, 1994, includes some multiplexing. The known system comprises several light-emitting elements and several optical receivers, operating simultaneously. The system is intended to achieve higher bit rate by information transfer through the atmosphere from the simultaneous use of several receivers and transmitters to create several spatially-divided information transmission channels (with number of channels equal to the number of receivers and corresponding number of transmitters interconnected with optical beams).
The known free-space optical communication system with spatial mutliplexing, however, suffers from several disadvantages. First, the system has limited application and complex design when the number of channels is large, because the maximum achievable number of channels in this system is equal to the number of transmitters or the number of corresponding receivers. Also, to separate the channels, it is necessary for each receiver to receive only the transmitter signal intended for or addressed to this receiver, which makes the system exceedingly complex. To provide division of channels, each transmitter should have a lens forming a narrow optical beam, and the receivers should be placed far enough from each other to avoid overlapping of optical beams from different transmitters, caused by the beam divergence. This, in turn, causes an increased size of each optical communication device, if the number of channels is large. This also limits the communication distance, because each lens is forming optical beams of non-zero divergence, and the transverse dimensions of each beam, starting from some particular distance, will increase proportionally to the distance from the transmitter.
Thus, the known free-space optical communication system is inconvenient for creating of a large number of independent communication channels, which in turn limits the ability to increase the system capacity for servicing a large number of subscribers with large data-rate demands.
The known system also suffers from the drawbacks common for prior art free-space optical systems: a) requirement for clear line-of-site between communicating terminals; b) lack of flexibility in transmission capacity available to terminals with changing demand for bandwidth. Hence, there is a continuing need for a free-space optical communication system with increased capacity, providing flexible connectivity between multiple terminals not necessarily having line-of-site with all other terminals,xe2x80x94without requiring overly complex optical elements or other system components. To the extent that the system relies on multiplexing, any such system should implement the multiplexing in as simple and effective manner as possible.
The optical communication device and system, which are the subjects of this application, are intended to simplify the system design for a given number of communication channels. The system also is intended to provide an increase in transmission rate and the number of communication channels while retaining a relatively simple design and acceptable size. In addition, the system provides increased reliability of communication and allows connecting terminals that do not have a direct line-of-site therebetween. The system also allows a change in the number of channels connecting the transmitters and receivers in accordance with variable demand for the rate of information transfer to and from the various subscribers.
To achieve these results, the inventive system utilizes a repeater system with multiple receivers and transmitters servicing terminals or other nodes of the free-space communication network. The repeater system""s transmitters and receivers are arranged in groups, so as to form several spatially divided optical receiver modules and spatially divided optical transmitter modules. The repeater system is installed between a transmitting node having several optical transmitters and a receiving terminal with several optical receivers. Each multi-beam optical receiver module of the repeater system connects via a separate optical beam with one of the transmitting terminal transmitters. Each multi-beam optical transmitter module of the repeater system connects with one of the receiving terminal receivers via a spatially separate beam. The repeater system components are spatially separated far enough from each other to prevent optical radiation of the beams connecting: a) different transmitters of the transmitting terminal with the receivers of different repeaters, and b) the transmitters of different repeaters with different receivers of the receiving terminal, xe2x80x94from entering the receivers not designated for this radiation.
The communication channels between the receiver modules and the transmitter modules of the repeater system may provide transmission, amplification, and, if necessary, processing of the received signal before transmitting to the receiving terminal receiver. The channels preferably are reconfigurable, for example, via selective links though one or more switches or routers.
One distinguishing feature of the invention is positioning of a repeater system between the communicating terminals, where the repeater system includes several receiver modules and several transmitter modules spatially separated far enough from each other to prevent optical radiation of the optical beams connecting the transmitters and receivers of the repeater modules and the network nodes or terminals from entering the receivers not designated for this radiation. This feature makes the transmitting and the receiving terminals with multiple transmitters and receivers compact and eliminates the need for placing the transmitters and the receivers far from each other to avoid overlapping of the optical beams.
The division of radiation emission of transmitters pointed at different spatially separated receiver modules of the repeater system is achieved due to angular separation of such beams. Each beam emitted by the transmitting terminal is pointed at the corresponding repeater receiver module and enters only this receiver module, thus providing a spatial division of channels created by these beams.
The beams directed from different transmitter modules of the repeater system towards one receiving terminal enter the terminal at different angles due to the spatial separation of the transmitter modules. This arrangement provides an angular form of spatial division. In particular, the repeater system transmitter modules are sufficiently separated from each other such that the arrival or incidence angles of the beams for one terminal allow for use of simple optical elements in the terminal to direct the beams separately to different optical receivers of the terminal, without their overlapping on such user terminal receivers. At the same time, the distances between the receivers of the receiving terminal in this system are not of any significance, which allows for use of a relatively compact design for the user terminal.
The angular selectivity of the receivers in the terminal nodes and/or in the modules of the repeater system, described above may be implemented, for example, by means of their arrangement in the vicinity of the focal surface of the receiving objective (lens). The beams entering the objective""s receiving aperture at different angles, are focused in different areas of the focal surface of the receiving objective (lens). Coupling the photodetectors of the receivers to locations in the vicinity of such focal areas, corresponding to different beams, provides a separate reception of these beams by the elements of the receivers.
The system also provides an increase of the maximum communication range. This results from the amplification of information-carrying signals in the repeater system, as well as from reduced signal scintillation caused by turbulence of the atmosphere at the shorter distances between the transmitting terminal and the repeater system, and between the repeater system and the receiving terminal, compared to the overall distance between the transmitting and the receiving terminals.
Another distinguishing feature of the invention relates to the use of multi-beam repeater transceiver modules, each equipped with several receivers having angular selectivity and thus able to separately receive the light beams entering the module at different angles, and each equipped with several transmitters arranged to output radiation beams at different angles. This makes it possible to have several transmitting terminals and several receiving terminals. One of several transmitters of each transmitting terminal is connected via a separate optical beam with one of the receivers of each of several of the repeater modules. One of the transmitters of two or more transmitter modules of the repeater system is connected via a separate optical beam with one of the receivers of the receiving terminal. A separate reception of beams from different transmitting terminals by different receivers of the repeater system is provided due to the angular selectivity of the repeater module receivers, as well as due to the fact that beams from different transmitting terminals are entering the repeater modules at different angles. Implementation of separate reception of beams from different transmitting terminals requires separation of such terminals in space sufficient for the repeater system receivers with angular selectivity to resolve signals arriving from different nodes or terminals.
Another distinction is that the repeater system may be equipped with routing or switching networks, which make it possible to connect each of the repeater module receivers with each of the repeater module transmitters. This allows the system operator to change the number of the beams connecting different transmitting and receiving terminals, to meet the demand for information transmission rate between particular terminals. This also provides higher reliability of the system as the beams blocked by obstacles may be replaced with the ones having clear paths.
The inventive system may provide dual exchange of information between two or more terminals or other types of nodes, each equipped with the transmitters and the receivers. To achieve this, the repeater system would include two or more modules equipped with receivers and transmitters, i.e. to form transceiver modules. An additional advantage of such a structure is that, one repeater system module may provide dual exchange of information between any two or more nodes. In some cases, information received via one beam at a transceiver module may be retransmitted from the same module on a different beam, i.e., going to a destination node or terminal. This creates the additional flexibility to allow the system to connect multiple terminals separated by an obstacle blocking the lines-of-site between the terminals.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following and the accompanying drawings or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.